1. Field of the Invention
The present invention relates to a process for preparing vinyl chloride homo- and copolymers in the form of latices comprised of monodisperse particles, and, more especially in the form of latices comprised of coarse particles, by polymerization of the corresponding monomer(s) in aqueous emulsion. This invention also relates to the final product latices and to the homo- and copolymers recovered from such latices.
2. Description of the Prior Art
It is known to this art that latices of vinyl chloride polymers are aqueous dispersions of particles of such polymers, the diameters of which typically range from 0.01 to 5 .mu.m.
By the term "monodisperse" particles as utilized herein, are intended particles whose particle size distribution is characterized by a polydispersity coefficient not exceeding 0.06.
By "coarse" particles are intended particles whose mean diameter is at least 0.5 .mu.m.
Latices containing monodisperse particles are of interest to the art because of the fact that prior knowledge of the mean diameter of such particles and the polymer content thereof makes it possible, by mathematical calculation, to accurately determine the value of the total surface area of the interface between the particles and the aqueous phase.
Latices which can be employed as biological reactants enabling certain pathological disease states to be detected are obtained by graft polymerization of vinyl monomers containing functional groups such as, for example, --COOH, --SO.sub.3 and --NH.sub.2 in the presence of the aforementioned latices.
The latices may, furthermore, be used as standards in methods of measuring diameters utilizing all scientific apparatus which includes devices for measuring diameters and operating in this range of values (particle size distribution analysis equipment, filtration equipment, calibrated pore membranes).
Upon admixture with one or more liquid plasticizers, the homo- and copolymers derived from the latices are obviously capable of being employed for the preparation of plastisols which can be utilized in an appropriate manner in a great number of techniques such as, especially, coating, rotational molding, dip coating and spraying. It is possible to prepare plastisols with accurately determined Theological behavior by use of a mixture of at least two homo- or copolymers derived from latices, at least one of which contains monodisperse particles having a mean diameter greater than 0.8 .mu.m.
Techniques for preparing vinyl chloride homo- and copolymers in the form of latices containing coarse monodisperse particles, by polymerization of the corresponding monomer(s) in aqueous emulsion, are also known to this art. These techniques entail a number of polymerization stages, each stage being carried out under moderate stirring, in the presence of at least one emulsifier and at least one water-soluble initiator. In the first stage, a latex of vinyl chloride homo- or copolymer containing particles having a mean diameter generally ranging from 0.1 to 0.3 .mu.m is prepared. It is known that the particle size distribution of the particles of the resulting latex depends essentially upon the nature of the emulsifier employed and on the initial rate of polymerization. Thus, for example, when employed as an emulsifier, sodium versatate produces, all other parameters remaining unchanged, latices whose particles have a particle size distribution with a lower polydispersity index than sodium myristate or sodium laurate. It is also known that polymerization at a high initial rate produces, all other parameters remaining unchanged, latices whose particles have a particle size distribution with a lower polydispersity coefficient than a polymerization at a low initial rate. The disadvantage of polymerization at a high initial rate is that the polymerization rate, being autoaccelerated, is no longer compatible, at a high degree of conversion of the monomer(s), with the heat transfer capacities of the reactor in which the polymerization takes place. Consequently, even when the operating conditions of polymerization promote the production of latices containing particles which have a particle size distribution with a lower polydispersity coefficient, the said polydispersity coefficient is generally greater than 0.1. In the second stage, a seeded emulsion polymerization is carried out in the presence of the latex originating from the first stage, as the seed material. A latex is obtained in which the particles have a mean diameter generally ranging from 0.3 to 0.9 .mu.m, higher than that of the particles of the seed material, and having a particle size distribution with a lower polydispersity coefficient, but which is still, in the majority of cases, greater than 0.08. Consequently, in order to obtain a latex containing coarse monodisperse particles, it becomes necessary to carry out more than two stages of polymerization, by employing as seed material, at each stage, from the second stage forwards, the latex originating from the preceding stage and employing a high proportion of seed polymer. Not infrequently, it becomes necessary to conduct four or five polymerization stages, and this entails protracted and costly operations, with loss of product at each stage.
In the article Plastics and Rubber: Materials and Applications, "Mechanism of emulsifier-free emulsion polymerization of vinyl chloride", February 1980, pages 21 to 24, J. Laaksonen and P. Stenius have described a technique for polymerizing vinyl chloride in aqueous emulsion which consists of operating in the complete absence of emulsifier, and in the presence of an alkali metal or ammonium persulfate as a water-soluble initiator, employed in a large quantity. In this article, the authors describe the means which enable a polyvinyl chloride latex containing monodisperse particles having a mean diameter ranging from 0.380 .mu.m to 0.535 .mu.m to be produced in a single stage. These means consist of the addition to the reaction medium, before polymerization, of a salt such as, for example, potassium chloride or calcium chloride, the cation of which, designated a counterion, is of a specified nature and is present in a specified amount. The mean particle-diameter of the latex obtained is a function of the intensity of stirring of the reaction mixture. It is proportionally higher, all other parameters remaining unchanged, the greater the intensity of stirring.
It has now been found, however, that the technique described by Laaksonen and Stenius suffers from the following disadvantages: the molecular weight of the polymer obtained depends on the intensity of stirring of the reaction mixture; it is proportionally higher, all other parameters remaining unchanged, the greater the intensity of stirring. The latices prepared by seeded emulsion polymerization in the presence of a latex prepared according to this technique, as a seeding material, contain nonspherical particles, and this renders them unsuitable for many applications.